Enhancement mode field effect devices are normally-off devices, in which no current flows between the source and drain contacts unless a positive voltage is applied to the gate. Such field effect devices include, for example, high electron mobility transistors (HEMTs) that have a two-dimensional electron gas (2DEG) between two active layers (e.g., between an GaN layer and a AlGaN layer) deposited on a silicon carbide (SiC) or sapphire substrate. This two-dimensional electron gas layer is a result of spontaneous and piezoelectric polarization leading to charge separation within the materials.
In some field effect devices, the two-dimensional electron gas is present at zero gate bias as a result of the characteristics of the materials. Most GaN field effect transistor (FET) devices with contacts on top of the AlGaN layer are normally-on devices due to the fact that certain a thickness of AlGaN barrier is needed for high conductivity connecting the source and drain of a FET. A particular negative voltage, called the threshold voltage, must be applied to the gate to deplete the two-dimensional electron gas through capacitive coupling. By applying a negative voltage to the gate, the electron channel can be pinched off. This negative voltage is typically below some negative threshold voltage (Vth). These transistors operate in depletion mode (i.e., the channel has to be depleted to switch off the transistor). For some applications, such as power switching or integrated logic, a negative-polarity gate voltage supply is undesirable.
In contrast to depletion mode devices, field effect transistors with a positive threshold voltage are normally-off devices such that at zero gate voltage, no channel is present to conduct current. These transistors operate in enhancement-mode, which can be useful for normally-off power switches, digital electronics applications, and high efficiency RF applications.
To make a normally-off device, the channel under the gate contact can be interrupted selectively while at the same time preserving a two-dimensional electron gas density in the other regions. A positive threshold voltage applied to the gate will then induce a two-dimensional electron gas under the gate contact, allowing current to flow between source and drain.
Various techniques have been proposed to obtain enhancement mode field effect devices. For example, one proposed structure uses an ultrathin InAlN/AlN barrier between the gate and the channel. See Ostermaier et al., “Ultrathin InAlN/AlN Barrier HEMT With High Performance in Normally Off Operation,” IEEE Electron Device Letters, vol. 30, no. 10, pp. 1030-1032 (October 2009). As disclosed in this article, the enhancement mode HEMT structure includes a 1-nm lattice-matched InAlN barrier with an additional 1-nm AlN interlayer, which results in a 2-nm thin barrier over a GaN buffer. The structure is capped with a highly doped n++ GaN layer. An enhancement mode device is achieved by selectively etching the GaN cap layer to form a recess for the gate metallization and leaving a 2-nm barrier between the gate and the channel.